Polycarbonate resins and polymethyl methacrylate resins have been widely used as substrate materials for optical recording such as optical disks and optical cards because they are excellent as optical materials. Out of them, polycarbonate resins are widely used as substrate materials for optical disks as they have excellent transparency, heat resistant stability and toughness.
In recent years, the recording density has been increasing due to technical progress typified by an increase in the capacity of an optical (opto-magnetic) recording disk, the development of DVD and the development of a blue laser. The thickness of a disk substrate is reduced from 1.2 mm for CDs to 0.6 mm for DVDs, and a thinner disk substrate is required to increase the recording density of an optical disk. However, in the production of an optical disk substrate having a thickness of 0.35 mm or less by injection molding, it has been difficult to obtain a substrate which is satisfactory in terms of the filling of a resin into the periphery of a substrate, thickness uniformity on inner and outer sides, and transferability of pits and grooves because the resin is cooled very quickly. Although it is possible to obtain a 0.3 mm-thick substrate which is satisfactory in terms of these characteristic properties by changing a mold and molding conditions, the value of birefringence becomes extremely large as molecular orientation by a flow of the resin at the time of molding is hardly eased. The optimization of the production conditions has its limit in the reduction of birefringence. To reduce the birefringence, amorphous polyolefins have been proposed by Zeonex, Zeonor (Japan Zeon Co., Ltd.) and Arton (JSR Corporation). Since these polyolefins have a small photoelastic coefficient, a 1.2 mm-thick polyolefin substrate having lower birefringence than a polycarbonate substrate can be obtained. However, a 0.6 mm-thick polyolefin substrate has a birefringence about ½ that of a polycarbonate substrate and a 0.3 mm-thick polyolefin substrate has almost the same high birefringence as a polycarbonate substrate. When the thickness is 0.1 mm, it is difficult to obtain a substrate having a predetermined outer diameter by injection molding and the stiffness of the substrate is reduced, whereby it is difficult to remove the substrate from a stamper and take it out by a robot.
In contrast to this, a method of transferring a stamper pattern to a film by heat and pressure by contacting the film to the stamper has been proposed. JP-A 1-113224 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) proposes a method of pressure molding a thermoplastic resin film or sheet while a DC field is applied to the film or sheet at a temperature higher than the glass transition temperature of the thermoplastic resin material and discloses that birefringence nonuniformity is reduced by the above method. JP-A 4-270633 proposes a method of heating with a film-like heater sandwiched between a heating plate and a stamper and a method of letting pass a combination of the stamper and a thermoplastic resin between heating and pressure rollers to improve nonuniformity in birefringence. According to these methods, temperature elevation and cooling times are shortened to improve productivity but a stamper pattern is not partly transferred. Even if this defect cannot be observed with the naked eye, a fine defect as large as several nanometers to several micrometers can be observed through a microscope or AFM (atomic force microscope) and becomes a defect of a medium. A continuous transfer method which uses rollers and is different from these methods using a leaf type press is also proposed. JP-A 5-269845 proposes a method of transferring a pattern by sandwiching a molten resin sheet between a roll stamper having a preformat pattern on the surface and a mirror surface roller. As the resin sheet is continuously supplied in this method, the productivity is high and the number of partial transfer defects as described above is relatively small. However, force applied to the resin sheet in the longitudinal direction differs from that in the transverse direction and it is extremely difficult to maintain dimensional uniformity. JP-A 11-345436 proposes a method of correcting the position and shape of a resin sheet relative to a stamper in anticipation of its deformation. However, the size and shape of the sheet are slightly changed by winding conditions and pressing conditions and it is difficult to control these completely.
The optical recording density is expected to become higher and higher from now on and the appearance of an optical recording substrate which can be used with a laser light source having a short wavelength and an optical head having a high numerical aperture is desired. Particularly, it is urgently desired to increase the recording density of an optical disk having a high access speed and capable of high-density recording by using laser light having a short wavelength. A 0.35 mm or less thick substrate having transferred grooves and pits is the most promising to realize that. In the prior art, a substrate having the above thickness and satisfying the above requirements cannot be obtained by injection molding or press molding.